1. Field of the Invention
The present invention relates to resonant driving circuits for piezoelectric actuators for vibrating vibrators.
2. Description of the Related Art
Piezoelectric actuators often include electrodes and piezoelectric materials, such as lead zirconate titanate (PZT) ceramics. The piezoelectric actuators are basically voltage driven devices because voltage applied thereto causes the piezoelectric actuators to mechanically deform. The piezoelectric actuators are often driven to resonate. The phrase “to resonant drive” indicates a driving method capable of resonating a piezoelectric apparatus, such as a piezoelectric actuator or a structure including the piezoelectric actuator, at a specific frequency determined in accordance with the mechanical shape/size thereof to yield a large deformation that does not result from a general voltage application method.
To resonant drive a piezoelectric apparatus, alternating voltage having a frequency substantially equal to the resonant frequency of the piezoelectric apparatus is applied to the piezoelectric apparatus. More specifically, for example, an oscillation circuit arranged to generate alternating voltage of the resonant frequency may be connected to the piezoelectric apparatus through a power amplifier.
However, since the resonant frequency differs for each of various different piezoelectric apparatuses because of process variations of the piezoelectric apparatuses and insufficient accuracy of positions of piezoelectric actuators attached to vibrators, it is difficult to resonant drive the piezoelectric apparatuses simply by applying an alternating signal of a predetermined fixed frequency to the piezoelectric apparatuses. The frequency of the applied alternating voltage may be adjusted for each piezoelectric apparatus. However, since the resonant frequency of the piezoelectric apparatus greatly changes depending on temperature, stable resonant driving the piezoelectric apparatuses is still difficult even with such adjustment.
In the related art, self-drive circuits for resonant driving (hereinafter, referred to as self-resonant-driving circuits) have been proposed that automatically determine the resonant frequency of piezoelectric apparatuses and generate alternating signals of the determined frequency. In one example of such a self-resonant-driving circuit, a piezoelectric actuator includes electrodes and a terminal for detecting an amount of deformation to define a three-electrode piezoelectric actuator. A self-resonant-driving circuit is configured to receive a driving signal that is applied to the piezoelectric actuator and then positively fed back through the deformation-amount detecting terminal. That is, this example is a method for driving and controlling the piezoelectric actuator so that the maximum deformation of the piezoelectric actuator is obtained.
However, since such a three-electrode piezoelectric actuator requires a complicated manufacturing process, the manufacturing costs are increased. Additionally, particularly in a piezoelectric actuator having large vibration amplitude, a large distortion is caused between a driven portion that greatly deforms and a non-self-deforming portion including the deformation-amount detecting electrode. The large distortion damages the piezoelectric actuator.
When a two-electrode piezoelectric actuator is used that is tolerant to the large distortion and does not includes the deformation-amount detecting electrode, a circuit configuration can be used in which the piezoelectric actuator is included in a resonant system of a driving circuit so that the frequency of the alternating voltage applied to the piezoelectric actuator is controlled to match actual resonant frequency of the piezoelectric actuator.
A self-resonant-driving circuit is described in the Magazine “Fuel Cell”, written by Kamiya Gaku, Kurihara Kiyoshi, and Hirata Atsuhiko, published by Fuel Cell Development Information Center, Apr. 30, 2009, VOL. 8, No. 4 2009, P148-151, FIG. 2. FIG. 1 is a diagram illustrating a basic configuration of a piezoelectric-actuator driving circuit described in the article. A current path of a piezoelectric actuator “a” includes a current detecting resistor R. The resistor R extracts a voltage signal proportional to current flowing through the piezoelectric actuator “a”. An operational amplifier OP supplied with the positively fed back voltage signal drives the piezoelectric actuator at a frequency where a voltage/current phase difference of the piezoelectric actuator is substantially equal to 0°.
Since a self-resonant-driving circuit for resonant driving an element with a resonance characteristic, such as a piezoelectric actuator, has a complicated circuit configuration, one terminal of the piezoelectric actuator is connected to ground as illustrated in FIG. 1. When an increased vibration amplitude is desired for the piezoelectric actuator illustrated in FIG. 1, higher power supply voltage is needed.
More specifically, to vibrate the piezoelectric actuator at large amplitude, alternating voltage generated from direct-current (DC) voltage that is greater than the power supply voltage of an oscillation circuit is typically applied to the piezoelectric actuator. The higher DC voltage is generated from the power supply voltage of the oscillation circuit.
Including a DC to DC converter in a driving circuit to boost voltage increases the number of components of the driving circuit. Additionally, the use of high voltage components increases the size of the driving circuit. Furthermore, since the many components of the driving circuit have to be resistant to high voltage, the cost thereof increases.